Fresnel lens is a thin lens. A Fresnel lens may be obtained by segmenting a continuous curved surface of an ordinary lens into a plurality of segments and arranging, after reducing the thickness of each segment, the segments on a same plane or on a substantially smooth curved surface. The refraction surface of the Fresnel lens is generally discontinuously stepped or dentate. In the present disclosure, the curved surfaces (non-smooth surfaces) of the lens are referred to as refraction surfaces.
FIG. 1 shows an ordinary configuration of the Fresnel lens. In FIG. 1, the dash line represents the center of the curved surface. The original curved surface 101 of an ordinary lens 100 may be segmented into a plurality of concentric lens rings 201. After the thickness of each lens ring is reduced, the plurality of lens rings may be arranged on a same plane to form a Fresnel lens 200. Such discontinuous refraction surface evolved from the original curved surface may be referred to as Fresnel refraction surface. Since the refraction of light occurs on the curved surface of the lens and is independent of the thickness of the lens, the Fresnel refraction surface theoretically has optical performance similar to that of corresponding original curved surface, but with greatly reduced thickness. The reduction in thickness can reduce the absorption and attenuation of light energy, which is an important advantage of the Fresnel lens in many applications.
The Fresnel refraction surface generated from one original curved surface may be referred to as one Fresnel unit. A Fresnel unit may be described using five groups of basic parameters: center location, area, focal length, refraction surface shape, and locations and number of segmentation rings.
For simplicity, in the present disclosure, the side on which the Fresnel refraction surfaces are arranged is referred to as “tooth side”, the other side which is relatively smooth and flat is referred to as “back side”, and the Fresnel lens which has a tooth side on one side and a back side on the other side is referred to as “single-sided Fresnel lens”.
The Fresnel lens can not only be used to focus light signals (for example, infrared) to facilitate the detection of the sensor (for example, the passive infrared detector “PIR” shown in FIG. 2), but also be used to focus other electromagnetic wave signals, such as microwaves, radio, X-rays and Gamma rays, etc. Therefore, the “light” herein may refer to electromagnetic wave in any spectrum band of the entire electromagnetic spectrum.
The focus range of single Fresnel unit is limited. In order to increase the signal sensing range, it is also possible to arranging a plurality of Fresnel units on the tooth side. The tooth side on which only one Fresnel unit is arranged may be referred to as “simple Fresnel refraction surface”. The single-sided Fresnel lens using such tooth side may be referred to as “single-sided simple Fresnel lens”. Correspondingly, the tooth side on which two or more Fresnel units are arranged may be referred to as “composite Fresnel refraction surface”, and the single-sided Fresnel lens using such tooth side may be referred to as “single-sided composite Fresnel lens”.
The back side of the single-sided composite Fresnel lens is generally a macroscopic surface, such as plane, coaxial surface (including rotation surface, such as sphere, ellipsoid, cylindrical surface, parabolic cylindrical surface, hyperbolic cylindrical surface and high order polynomial surface, etc.), multi-plane surface formed by splicing a plurality of planes, and trapezoidal table surface, etc. FIG. 2 shows the configuration of several single-sided composite Fresnel lens, where the dash lines represents the light paths passing through the centers of the Fresnel units. In FIG. 2(a), the tooth side includes three Fresnel units arranged horizontally, and the back side is a plane (rectangular). In FIG. 2(b), the tooth side includes five Fresnel units, one of which is located at the center and the other four are distributed around, and the back side is a plane (circular). In FIG. 2(c), the back side is a circular cylindrical surface. In FIG. 2(d), the back side is a sphere. In FIG. 2(e), the back side is a multi-plane surface formed by splicing three planes. In FIG. 2(f), the back side is a trapezoidal table surface.
Currently, the existing Fresnel lenses are generally single-sided simple Fresnel lens and single-sided composite Fresnel lens. The performance of these lenses is greatly limited. Therefore, further research and development to Fresnel lens system are needed.